Present state of the art flight data recorders may be classified into the major categories of electromechanical recording devices and solid-state memory devices. The electromechanical recording devices represent the majority, being used on both civil and military aircraft. They may be either of two types: analog foil recorders or digital magnetic tape recorders.
In the analog foil recorders the parameter data to be recorded is in analog form, and is continuously recorded by engraving the value of the parameter on a metallic foil record with scribers. The scribers are actuated by the sensed parameter magnitude and time correlation is provided by a constant speed drive of the foil, which is marked by a separate time scribe. These recorders can record only a limited number of flight parameters, but provide continuous recording for extended time intervals e.g. over 400 hours for commercial aircraft. Their disadvantages lie in their large size and weight, the limited parameter recording, their low reliability due to the large number of mechanical parts, and the need to remove and replace the foil cartridge at the end of the recording interval. This combined with high periodic maintenance costs make them impractical for smaller civil and military aircraft in which the flight profiles (recording time intervals) are much smaller than that required in the commercial field.
In the digital magnetic tape recorders, or Digital Flight Data Recorders (DFDR), the sensed analog parameters are converted to a digital format and continuously recorded on magnetic tape. The digital data is in a serial binary form from an intermediate data acquisition unit (DAU). The recording capacity of these DFDR type recorders is 25 hours. Since they are also electromechanical they suffer from the same degradation in reliability common to the metal foil recorders, and are also large and bulky due to the tape storage and motor drive elements required by the system. As such they also have limited practical use on small civil and military aircraft.
The other type of flight recorders, e.g. the newer type solid-state flight data recorders, eliminate the mechanical disadvantages of the electromechanical devices, and are made possible by the rapid progress in technology of integrated circuit (IC) memory devices. These memory devices used include both volatile memories (lose the stored data when power is removed) such as RAMs and CCDs (charge coupled device) and nonvolatile memories such as a ROM, PROM, EAROM (electrically alterable read only memory), E.sup.2 ROM (electrically erasable programmable read only memory) and magnetic bubble memory devices. Only the nonvolatile devices that can be written over in situ are suitable for recording flight data in a permanent storage capability for later readout by ground based equipment.
While providing higher reliability and lower size and weight than the electromechanical recorders, the present solid-state recorders have a limited ability for continuous recording over extended time intervals. This results from the limitation on bit storage capacity of the solid-state memory devices. For a serial binary, twelve bit word received at 64 words per second, the signal bit times are 768 BPS, 46.08 KBPM and 2.8 MBPH; a 25 hour continuous recording interval requires 70 megabits of storage. Even with magnetic bubble memories, which provide the highest bit storage density per unit cost among nonvolatile memory devices, the present state of the art bit storage capacity is on the order of one megabit per device. As such, a 25 hour recording interval requires 70 bubble memories, which is impractical.
In order to make the solid-state recorders practical, a recording scheme is required which reduces the necessary solid-state memory sizes. Of course this must be done without sacrificing the integrity of the recorded data so as to allow for an accurate reconstruction of data during an accident investigation procedure.